What is meant by 'period' and 'group' in the periodic table?

In the periodic table, 'period' refers to the horizontal rows, while 'group' refers to the vertical columns.

In the periodic table of elements, a 'period' is a horizontal row. The elements in a period are not similar in properties. First period contains only two elements - hydrogen and helium, while the second and third periods have 8 elements. Elements of the fourth and fifth periods have 18 elements. The sixth and seventh periods have 32 elements because the two bottom rows that are separated from the rest of the table (the lanthanides and actinides) belong to these periods. The length of each period is determined by the number of electrons that can occupy the sublevels being filled in that period.

On the other hand, a 'group' in the periodic table refers to a vertical column. Elements in a group have similar physical or chemical characteristics of the outermost electron shells of their atoms (i.e., the same core charge), as most chemical properties are dominated by the orbital location of the outermost electron. There are 18 numbered groups in the periodic table; the f-block columns (between groups 3 and 4) are not numbered. The elements in a group have similar properties to each other. For example, the alkali metals lie in group 1 and share similar properties, such as high reactivity and the tendency to lose one electron to arrive at a noble-gas electron configuration.

In summary, the periodic table is organised into periods and groups to classify elements according to their shared properties and atomic structure. This arrangement helps chemists understand and predict the behaviour of elements, making it an essential tool in the study of chemistry.